An area covered by a cellular telecommunications network is divided into a plurality of cells. Each of these cells has a base station arranged to transmit signals to and receive signals from mobile stations located in the cell associated with the respective base station. Mobile stations will be in active communication with the base station associated with the cell in which the mobile station is located.
A modulation system has been developed for use within a cellular network using the Global System for Mobile communication (GSM) standard which enhances the rate at which data is transferred between the mobile stations and the base stations. This modulation scheme is called Enhanced Data rate for GSM Evolution or EDGE modulation. EDGE modulation is a known modulation scheme so will not be explained in detail hereinafter. It is sufficient to note that EDGE modulation modulates digital data using both phase and amplitude information. To reduce errors at the receiver, the transmitted signal is substantially similar to the modulated signal. Therefore the phase and amplitude errors introduced between the EDGE modulator and the transmitting antenna should be kept to a minimum. Power amplifier circuitry may introduce such phase and amplitude errors.
FIG. 2 shows a conventional power amplifier 12. The power amplifier 12 may have at its input 8 a modulated radio signal 10. The input radio signal 10 may be a modulated digital signal. The power amplifier 12, when operated beyond saturation produces intermodulation distortion products 17 at its output 18 as well as the desired carrier signals 14. The intermodulation distortion products 17 are produced on either side of the desired carrier signals 14. Intermodulation distortion products 17 are caused by the power amplifier 12 not acting as a linear amplifier, which occurs when the power amplifier is operated beyond saturation.
The intermodulation distortion produces frequencies at multiples of the carrier frequency of the desired signals. These frequencies 17 tend to be lower in signal strength than the desired carriers by an amount which is referenced 16 in FIG. 2. These intermodulation distortion products 17 increase the spectral space occupied and are therefore undesirable.
One way to reduce the intermodulation distortion products, is to operate the power amplifiers as linear amplifiers. The power amplifiers would be operated so that there is a substantially linear relationship between the input signal power and the output signal power.
FIG. 3a shows a graph of output signal power against input signal power for a typical power amplifier. The linear region 20 in which the power amplifier should be operated to reduce intermodulation distortion is not as efficient as if the power amplifier were operated at a point 24. Point 24 is called the P1 point and the amplifier operates non linearly beyond this point. The P1 point is a figure of merit and is defined as the point where there is 1 dB of compression 28 and is the point at which the power amplifier is most efficient. In other words, the P1 point of the power amplifier is the point where the actual output power of the power amplifier is 1 dB less than the expected output power of the power amplifier if it were operated linearly as indicated by the dashed line 26. The linear region 20 extends to a point 27 where the power amplifier begins to saturate. Saturation means that the power of the output signal is no longer linearly related to the power of the input signal. Once the power amplifier is operated in the saturation region, a large increase in input signal power provides a small increase in output signal power. This is shown in FIG. 3a by the line referenced 22.
FIG. 3b shows the relative phase of an output signal of a power amplifier against input signal power. The phase of the output signal relative to the input signal is constant until point 31. The relative phase of the output signal decreases non linearly for an increasing input signal power beyond point 31. Point 31 may or may not correspond to point 27 of FIG. 3a. 
As EDGE modulation requires accurate transmittal of both amplitude and phase information, it is important that the power amplifiers are operated in the linear region 20 meaning a reduction in power amplifier efficiency. A typical power amplifier will be “backed off” by around 6 dB. This means that the power amplifier is operated around 6 dB below the P1 point 24 to ensure that the power amplifier operates in the linear region 20.
Reference is made to FIG. 3c which shows the efficiency of the power amplifier against the input signal power. As can be seen, the greatest efficiency is achieved when the power amplifier is operating non-linearly, in the region 32. If the power amplifier is operated in the linear region, more power is required to amplify a signal by a given value. The power amplifier consumes more power and the size and cost of the power amplifier is increased. Furthermore there will be increased heat dissipated by the power amplifier which may require extra cooling elements.
To allow operation of an amplifier with reduced intermodulation distortion at the P1 point, linearisers have been developed and are known in the art. A lineariser is placed before the power amplifier in the signal path and therefore preconditions the input signal before passing it to the power amplifier. A typical lineariser power characteristic 34 is shown in FIG. 4a. FIG. 4a shows output signal power against input signal power for a lineariser. As can be seen the lineariser has a linear power characteristic 36 up to the point 27 where the power characteristic begins to curve upwards. In other words the gain of the lineariser increases as the power of the input signal increases beyond point 27 and increases in such a way as to substantially oppose the typical power amplifier power characteristics. This is called gain expansion. A lineariser should be capable of producing about 5 dB of gain expansion to mitigate the non linear amplitude problems associated with power amplifiers beyond saturation as discussed previously. The effect of the lineariser is to increase the effective range over which the power amplifier is linear but allows the power amplifier to operate in its more efficient non-linear range.
FIG. 4b shows the relative phase characteristics 40 of a typical lineariser against input signal power. When the lineariser is operated with an input signal power less than the input signal power corresponding to point 31, the relative phase characteristic of the lineariser is substantially flat. The output signal has a substantially similar phase to the input signal. After point 31 however the lineariser has an increasing relative phase characteristic 44. The increasing relative phase characteristic produced by the lineariser is non linear and is such that it substantially opposes the decreasing relative phase characteristics of the power amplifier. This means therefore that the output signal from a lineariser is a pre-conditioned signal so that when it is subsequently fed into a typical power amplifier, the described phase limitations on a power amplifier are reduced.
The known linearisers are made from substantially large, discreet components. The linearisers have a complex structure. The linearisers typically require temperature compensation. In addition, the lineariser components may need to be carefully aligned. This all makes the use of known linearisers difficult within small low power devices. This is especially true for integrated circuits and in particular for use within microwave monolithic integrated circuits (MMIC). MMICs are used within many modern circuits for example in satellite and mobile telephony technologies.